A humidification element using a water absorptive sheet material is generally used for a humidification device. With the humidification device, water is supplied to the humidification element, and air is humidified using evaporation of the water. Thus, the humidification element needs to excel not only in performance, but also in water resistance from a viewpoint of preventing deterioration with water because of long-term exposure to water. Also, it is required that the humidification performance of the humidification element does not deteriorate due to VOC contamination because volatile organic compounds (VOCs) may exist in indoor environment immediately after new construction and reconstruction of a house or building. As water absorptive sheet materials for the humidification element, a base material on which a hydrophilic coating film is formed, a base material subjected to hydrophilic treatment, and other similar materials are used commonly.
The hydrophilic coating film can be formed using any of various materials and methods. For example, it is generally known that a hydrophilic coating film made of silica excels in hydrophilic properties. On the other hand, silica is prone to adsorb contaminants such as volatile organic compounds (VOCs). Consequently, when a base material on which a hydrophilic coating film of silica has been formed is used for a humidification element, there is a problem in that although good humidification performance is available at an initial stage, contaminants and similar substances are adsorbed over time, resulting in reduced hydrophilic properties and thereby failing to provide sufficient humidification performance.
As a method for solving this problem, it is conceivable to increase a specific surface area by making the hydrophilic coating film of silica porous and thereby expose large quantities of hydroxyl groups existing in a surface of the hydrophilic coating film. Consequently, even if contaminants and similar substances are adsorbed in the hydrophilic coating film, since large quantities of hydroxyl groups are exposed, it is considered that hydrophilic properties can be maintained for a long period of time.
As a method for making the hydrophilic coating film of silica porous, for example, a method has been proposed that gives hydrophilic properties to a surface of foam metal by forming a hydrophilic silica film (sol-gel silica) obtained by sol-gel method on the surface of the foam metal (see Patent Literature 1). Here, the sol-gel method is a method of synthesizing silica and the like by putting tetraethyl orthosilicate, which is a silica precursor, into a gel state by heating and the like and causing the tetraethyl orthosilicate to undergo hydrolysis and polycondensation reaction under acidic or basic conditions to desorb alcohol. Also, as another method, a method has been proposed that scatters porous silica gel particles in a hydrophilic silica film (sol-gel silica) obtained by sol-gel method and thereby improves the specific surface area of the film (see Patent Literature 2). Furthermore, as another method, a method has been proposed that scatters porous silica gel particles in a binder made of resin and thereby improves the specific surface area of a film (see Patent Literature 3).